A hollow piston in accordance with the prior art is described in DE 196 20 167 C2. In this previously known hollow piston, an annular hole, which is manufactured in a metal-cutting manner by deep-hole drilling, exists between the peripheral wall and the central mandrel. This presupposes an outlay on manufacture which is labour and time-intensive and which gives rise to relatively high manufacturing costs. At the same time, it must be taken into account that the manufacture of this known hollow piston also takes place with a relatively high wastage of material, which comes about because of the incorporation of the annular hole in a metal-cutting manner. Furthermore, this hollow piston has a ball-joint part in the form of a joint ball which extends from the base section of the hollow piston in the opposite axial direction to the cavity.
DE 26 53 867 A1 describes a hollow piston and a process for manufacturing the said piston, which is formed from two separate hollow-piston parts, namely a first piston part comprising the base section, the peripheral wall and the ball-joint part, and a second piston part containing the cover and a central mandrel which extends from the said cover in one piece. In order to stabilise the connection between the first and second piston parts, the cover grips into the hollow piston with the aid of a peg-shaped extension on the free rim of the peripheral wall, and the free end of the mandrel grips into a recess in the base section, the two piston parts being connected to one another by soldering. Although, in this known hollow piston, the peripheral wall of the first piston part on the one hand, and the mandrel of the other piston part on the other are, in each case, formed onto the appertaining piston part in one piece by extrusion, two separate forming operations in two separate forming tools are nevertheless necessary in this known design.
The underlying object of the invention is to construct a hollow piston of the type initially indicated, in such a way that a stable mode of construction is achieved while guaranteeing simple and cost-effective manufacture, and also to indicate a corresponding manufacturing process.
In the case of hollow pistons according to the present invention, the peripheral wall and the central mandrel are formed onto the base section of the hollow piston without cutting. This not only avoids the wastage of material caused by deep-hole drilling in a metal-cutting manner, but also avoids grooves extending in the peripheral direction in the inner superficies of the peripheral wall and in the outer superficies of the mandrel, as a result of which substantially greater stability is imparted to the hollow piston, since the peripheral grooves quite considerably reduce the said piston""s resistance to bending loads. Moreover, the forming-on of the peripheral wall and central mandrel in one piece can be performed more simply and rapidly from the production engineering point of view, without the need to dispose of swarf.
It is particularly advantageous to form-on the peripheral wall and central mandrel by extrusion. By this means, not only is shaping achieved which can be performed in a simple and rapid manner, but also material fibres are obtained in the course of extrusion which are directed in the longitudinal direction of the piston, as a result of which the peripheral wall and mandrel have a particularly high moment of resistance to bending loads imparted to them and also, basically, have great strength, a fact which is likewise achieved through extrusion as a result of material and structural compaction.
Alternatively, the peripheral wall and the central mandrel may also be formed, within the scope of the invention, through the fact that the said parts consist, with the base section, of sintered material and are formed and sintered with the annular hole which extends between the peripheral wall and the mandrel. Likewise, as a result of this, not only is simple, cost-effective manufacture achieved, but it is also possible to make use of the advantage that a sintered material is suitable as an antifriction material, this being attributable to the fact that the pores present in a sintered material form lubricant pockets which guarantee good lubrication and relatively low wearing of the sliding surfaces.
The hollow piston according to the invention in accordance with claim 6 consists of two pot-shaped blank sections which are identical as regards the shape and size of their internal shape, that is to say, the shape of the halves of the cavity. As a result of this, it is possible to use identical tools and manufacturing measures, at least for designing the internal shapes of the hollow pistons. This substantially facilitates the manufacturing process, and the manufacturing costs can be lowered considerably.
It is particularly advantageous if the hollow piston is composed of two blank sections or prefabrication parts which are identical not only as regards their internal shape, but also as regards their external shape, and are thus identical. It is therefore necessary to manufacture only one type of prefabrication parts, which are assembled in a mutually opposed disposition and are connected to one another, in particular by welding, and which thus form a piston blank.
The subclaims contain features which contribute to the achieving of the object underlying the invention, make possible a further saving on material and permit a smaller structural length of the hollow piston or of the piston engine as a whole. This latter fact is achieved, in particular, through the fact that the ball-joint part extending from the base section of the hollow piston is formed by a ball socket, whose outer peripheral face can be utilised as the outer superficies and sliding face of the hollow piston, so that the piston guide can extend right over the said ball socket and, consequently, the axial structural length of the piston; engine can be reduced.